I do know that you can be in different A/B tests in different workspaces. Or different phased rollouts

Right, that is also a possibility. I do hope that this new version of reality gets scratched though. :)

There was a “What’s New” popup in the channel header when I clicked in:

In May 2021, we launched a new design of the Slack desktop app to simplify and organize details related to your conversations. If things look and feel a little bit different today, you're in the right place to learn more about what's changed.

Note: These changes are rolling out gradually. We appreciate your patience as we make them available to all customers.

Full details are here: https://slack.com/help/articles/1500009429362-Find-your-conversation-details-organized-in-one-place

Funny how they call it "one place" when it's literally two separate buttons on their own screenshot, where the second button keeps disappearing. :) Shame alternative clients aren't that great. At least, in my experience.

Slack has such a large user base now that all changes tend to roll out slowly on a per-workspace basis (and occasionally A/B-tested on a per-user basis. I’m in a bunch of Slacks and it’s always a bit weird to see UI changes and preferences slowly adjust over a period of weeks across all my workspaces.